Electrolytic furnace with lined cathode pots for the production of aluminum



June 14, 1966 J. SCHMITT ETAL 3,256,173

ELECTROLYTIC FURNACE WITH LINED CATHODE POTS FOR THE PRODUCTION OF ALUMINUM Filed Oct. 24, 1961 2 Sheets-Sheet 1 June 14, 1966 J. SCHMITT ETAL 3,256,173

ELECTROLYTIC FURNACE WITH LINED CATHODE POTS FOR THE PRODUCTION OF ALUMINUM 2 Sheets-Sheet 2 Filed Oct. 24, 1961 I l I IIIllIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII"I/IIIIIII/IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII /05 INVENTORS.

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United States Patent i 3 256 173 ELECTROLYTIC FURisAcE WITH LINED CATH- ODE POTS FOR'THE PRODUCTION OF ALUMI- NUM Johannes Schmitt and Hubert Wittner, Rheiufelden, Baden, Germany, assignors to Swiss Aluminium Ltd., Chippis, Switzerland, a joint-stock company of Switzerland Filed Oct. 24, 1961, Ser. No. 148,017

Claims priority, application Switzerland, Oct. 28, 1960,

12,091/ 60 5 Claims. (Cl. 204243) The present invention relates to a lining composition for cathode pots of electrolytic furnaces used in the production of aluminum.

Furnaces for the production of aluminum by electrolysis of a fluoride melt have a cathode pot, which general ly comprises an iron casing with an inner lining of firebrick, if desired in combination with insulating blocks, and this lining is itself faced with a carbon composition. On the ceramic lining of the base of the pot there are generally laid pre-baked carbon blocks in which there are embedded iron cathode bars, which project through the sidewalls of the cell. A carbon composition is generally also applied as a lining to the side walls. In some cases, the whole of the interior of the cathode pot, that is to say both the walls and the base, are lined with such a carboncomposition which is poured on the lining and rammed. An example of a lining composition of this kind is the following:

Particle Amount Component size, mm by Weight,

percent Anthracite 0 to 15 26 Pitch coke- 0.21 to 3 48 Pitch coke 0 to 0.21 12 Pitch (medium-hard) 14 We. have found that cathode pots lined with such carbon compositions have certain disadvantages:

I (1.) The lining of the walls is attacked during operation by the molten electrolyte, and becomes eaten away so that particles of carbon enter the molten electrolyte and contaminate it.

(2) In order to protect the lining from attack by the molten electrolyte, the furnace must be operated so care fully, (for exampleby maintaining a precise excess of aluminum fluoride in the electrolyte or by maintaining an accurately definedtemperature) that the side walls become encrusted to a certain extent with solidified elec- Furthermore, because furnaces are commonly operated discontinuously, it is not always possible to protect the carbon lining with solidified electrolyte so that the lining can still be attacked.

(3) Because ofthe comparatively high electrical conductivity of the carbon walls, lateral stray currents arise in the cell, even when the walls are encrusted, so that the current density on the flat cathode surface does not reach the desired level and considerable losses may occur.

(4) The stray currents mentioned above have the further effect that the passage of current from the carbon bottomto the cathode bars does not occur in an even and concentrated manner, 'sothat a somewhat higher potential difference between the carbon bottom and the cathode bar's may result; Amongst other factors, the pressure with which the iron cathode bars are pressed against the carbon composition bottom of the pot plays an importantrent flowing. Thus, if stray currents cause uneven electrical loading of the cathode, the bearing pressure against the carbon bottom, and hence the cathode potential drop, will be unfavourably influenced by the resulting uneven heating of the cathode bars. I

(5) The carbon composition is a comparatively good conductor of heat and this leads to the above-mentioned lateral encrustation of the side walls. Because it is necessary periodically to break in this hard crust which forms on the molten electrolyte in order to introduce fresh alumina, neither the excess of aluminum fluoride in the electrolyte nor the furnace temperature remains constant. As a result, the'lateral encrustation constantly alters, being sometimes increased and sometimes reduced. However, in order to achieve the most favourable current yield in such a furnace it is necessary that the molten beds of metal and electrolyte should remain as stable as possible. In practice, attempts are made to achieve an average encrustation between the two extremes by careful furnace operation; however this necessitates further skilled labour.

It has been proposed to line the walls of the electroytic cells with thin ceramic plates, for example of silicon carbide bound together by silicon nitride or with kaolin. Some wall linings produced from such plates have an intermediate heat-insulating layer between them and the iron cell-casing, for example of alumina. The bottom of the furnace pot is lined conventionally with carbon blocks, the intervening gaps being packed with an unbaked carbon composition. The disadvantage of these materials,

- which generally contain silicon carbide as the main component, is that the binding agent between the plates is attacked by the molten electrolyte. There is a further disadvantage that the plates cannot be laid sufliciently close together to prevent molten electrolyte from penetrating through the gaps in time, with the result that even when using such ceramic linings, a protective layer of solidified electrolyte is necessary.

The object of the present invention is to provide a composition for lining the walls of a cathode pot in an electrolytic furnace for the production of aluminum, which avoids or materially reduces the drawbacks of the prior art described above, which can be moulded into shape in situ and which is a poor conductor of heat and electricity.

According to the invention, we provide a lining composition comprising from 40 to by weight powdered Percent by weight.

Powdered silicon carbide 70-80 Powdered coke 15-10 Medium-hard pitch 15 to 10 The particle size of the silicon carbide is conveniently up to 6 millimetres and the particle size of the coke up to 22 millimetres. Any desired type of coke and pitch may be used, and the latter may be either in liquid or solid form. The composition is conveniently mixed at a temperature between 50 and 250 C.

It appears that the mechanical strength which can be achieved on moulding and heating a composition according to the invention depends on the mutual action of the pitch and the powdered coke. These constituents appear to form a sort of honeycomb structure, in the hollow spaces of which the silicon carbide is embedded .as a filler. The new composition is resistant in a reducing atmosphere both to the molten electrolytaand to molten 3 aluminum. Depending upon the amount of silicon carbide in the composition, its electrical conductivity is from to 15 times smaller than that of the carbon compositions used hitherto. Its heat-conductivity is lower to a similar degree.

A substantial advantage of composition according to the invention is that they can be worked like conventional carbon compositions into moulded linings. They also bind tightly to such conventional carbon compositions, so that a continuous composite lining entirely free of spaces can be formed with walls of a composition according to the invention and a base of a conventional carbon composition.

A further feature of the invention is an electrolytic furnace for the production of aluminum having a cathode pot at least partially lined with a composition according to the invention, and alternative constructions of such a furnace embodying the present invention are illustrated in the accompanying drawings, in which:

FIGURE 1 is a diagrammatic sketch showing a partial section through a furnace with self-baking anodes;

FIGURE 2 is a similar section to FIGURE 1, through a furnace with pre-baked anodes; and

FIGURE 3 is amodification of the furnace shown in FIGURE 2.

FIGURE 1 shows an 80,000 amp. furnace comprising a cathode pot 1 and self-baking, continuous anodes 2. The bottom and sides of the iron casing of the pot are lined in a conventional manner with one or more layers of firebrick 9 to improve the heat-insulation. Carbon blocks rest on the firebrick bottom 9, and these are joined together by means of a conventional carbon composition to form a carbon bottom 3. The side walls of the cathode pot are lined with'a layer 4 of a lining composition consisting of a mixture of 75% silicon carbide, 14% powdered coke and 11% medium-hard pitch. The upper edge of the side walls is covered with a conventional carbon composition 5, containing no silicon carbide. This serves to protect the silicon-carbide-containing composition from the combined attack of molten electrolyte and atmospheric oxygen which may cause gradual combustion of the carbon in the composition, and may thus lead to contamination of the electrolyte with silicon. Current is supplied to the bottom of the cathode pot through the carbon bottom 3 and iron cathode bars 6.

The furnace shown in FIGURE 2 is a 40,000 amp. furnace, which differs from the furnace shown in FIG- URE 1 in that it has pre-baked carbon anodes 11. The remaining reference numerals with the letter a refer to the same parts as are shown in FIGURE 1 with the same numerals but without the letter a.

The furnace shown in FIGURE 3 is essentially the same as that in FIGURE 2, except that it shows a modified construction of wall-lining, which illustrates an important feature of furnaces according -to the invention. In FIGURE 3 the parts similar to these shown in FIG- URES 1 and 2 are designated by the same numbers, ex-

cept that in FIGURE 3 the numerals are followed by the letter b.

It will be seen from FIGURE 3 that the lining 4b is so shaped that the lower part of the pot is appreciably narrower than the higher part. During production, molten .aluminum collects in this confined area and should be run off once its level reaches the top of it. By confining the space occupied by molten aluminum, the current yield is improved. As a result of the relatively rapid rise of molten aluminum during production, the separation between the lower part of the anodes and the level of molten metal tends to become less, so that the furnace voltage tends to fall correspondingly. In other words, the rate at which the electrodes are consumed is less than the rate at which the level of molten metal rises. Because the heat energy supplied to the furnace tends to fall, the furnace itself tends to maintain a comparatively low or constant temperatprg, tht s improving the current yield.

4 In FIGURE 3 the lining 5b is covered with an additional protective sheet 7 of steel.

An electrolytic furnace in which the cathode pot has walls lined with a composition according to the invention offers the following advantages:

(1) During operation, stable electrical and thermal conditions are readily maintained, and also ready access is afforded to the electrolyte for replenishing this.

(2) The current yield can be from about 3 to 10% higher than in furnaces with conventional walls. The potential between the carbon bottom of the cathode pot and the cathode bars where they leave the furnace is about 0.1 to 0.2 volt lower, which corresponds to a reduction of the specific energy consumption of about 1 to 2 kwh./ kg. aluminum.

(3) As the molten electrolyte does not penetrate into the new composition, the consumption of molten ingredients is lower than in conventional cells.

(4) Because wall-linings produced from the new composition are attacked neither by the molten electrolyte nor by molten metal and as a result of the poor heat-conductivity remain hot and not encrusted, the horizontal dimensions of the beds of molten electrolyte and metal remain constant. The furnace can therefore be operated under the most favourable conditions.

The lining compositions according to the invention,

because of their high content of silicon carbide, are initially 2 to 3 times as expensive as conventional carbon lining compositions. This disadvantage can, however, beovercome by recovering the silicon carbide from discarded linings. This can be effected by partial combustion of the carbon in the composition at 600 to 1300 C.

In the following claims, by honeycomb matrix is intended a matrix which is somewhat in the shape of or resembles a honeycomb structure but which does not necessarily have the regularity of a genuine honeycomb structure.

What is claimed is:

1. An electrolytic furnace for the production of aluminum having a cathode pot with side walls lined with a composition composed essentially of a mixture of 40 to by weight of powdered silicon carbide, 45 to 7% by weight of coke powder and 15 to 8% by weight of pitch, the coke powder and the pitch forming a honeycomb matrix, and the silicon carbide being in discrete particulate form and being embedded in said matrix.

2. A furnace according to claim 1, in which the sidewalls are so shaped that the lower part of the cathode pot where molten aluminum collects has a smaller cross sectional area relative to the upper part occupied by molten electrolyte.

3. A furnace according to claim 1, in which the surfaces of the side wall lining exposed to the atmosphere are protected by a carbon lining composition. I

4. A furnace according to claim 1, in which the sur-. faces of the side wall lining exposed to the atmosphere are protected by a steel plate.

5. A furnace according to claim 1, in which the surfaces of the side wall lining exposed to the atmosphere are protected by a carbon lining composition covered by a steel plate.

References Cited by the Examiner UNITED STATES PATENTS 1,287,849 12/1918 Booth 252-504 1,479,107 1/1924 Ohrnan 106--69 2,915,442 12/1959 Lewis 204-243 2,938,807 5/1960 Andersen 106-44 2,952,605 9/1960 Devarda 204-243 JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner.

B. JOHNSON, H. S. WILLIAMS, E. ZAGARELLA,

, Assistant Examiners, 

1. AN ELECTROLYTIC FURNACE FOR THE PRODUCTION OF ALUMINUM HAVING A CATHODE POT WITH SIDE WALLS LINED WITH A COMPOSITION COMPOSED ESSENTIALLY OF A MIXTURE OF 40 TO 85% BY WEIGHT OF POWDERED SILICON CARBIDE, 45 TO 7% BY WEIGHT OF COKE POWDER AND 15 TO 8% BY WEIGHT OF 